The cytochrome P450 CYP3A subfamily is an expanding group of enzymes that plays a pivotal role in the detoxication of drugs, environmental contaminants and other xenobiotics and can also activate a number of procarcinogens. As part of our continuing studies on the structure and function of the enzymes we are employing a homology model of human CYP3A4 based upon the known structures of bacterial P450 enzymes to interpret the results of site-directed mutagenesis studies. In our initial work, we concentrated upon interchanging residues of CYP3A4 with the related but less catalytically active CYP3A5 and we are now engaged in the search for surface residues potentially involved in the interaction between CYP3A4, cytochrome P450 reductase and cytochrome b5. To allow effective visualization of the putative binding surfaces of the proteins, we are dependent upon the interactive graphics workstations and MidasPlus software of the Computer Graphics Laboratory. In another ongoing study, we are expanding our comparison of CYP3A enzymes to include rat CYP3A9 which has a number of interesting and distinct catalytic properties. We hope to determine the structural basis for these properties using a combination of site-directed mutagenesis, homology modeling and substrate docking in a manner analogous to our studies on CYP3A4/5. MidasPlus should prove invaluable in planning and interpreting the mutagenesis experiments.